Potentiometric measuring probes, ionsensitive probes or redox probes, such as pH measuring probes, are used in the electrochemistry. The fundamental structure of such a measuring probe includes two electrochemical half cells, wherein one half cell is referred to as an indicating electrode, for example, a glass electrode with a sensitive membrane, and the other half is referred to as reference electrode or as a reference diaphragm. The potential difference between the two half cells is proportional to the parameter to be measured, such as the pH-value of a solution.
The indicating electrode as well as the reference electrode has a certain internal resistance. As long as the electrodes operate normally, both internal resistances will remain substantially stable. However, in the event of an electrode failure, like the breakage, leakage or malfunction of the electrode membrane or the reference diaphragm the internal resistance of an electrode will change significantly. Consequently, by measuring the internal resistances of the electrodes their condition can be monitored and analyzed.
In order to measure the resistances of the electrodes individually, a third electrode, a so-called solution ground electrode may be inserted into the solution to be measured. Then the internal resistance of the indicating electrode as well as the reference electrode is measured in reference to the solution ground electrode. The resistance of the indicating electrode is very high and can vary over a very large range, whereas the resistance of the reference electrode is usually relatively small but can also vary over a large range.
Further, the potential difference between the electrodes is relatively small and significant capacitances exist between the wires of the connecting cable. All these factors influence the measurements of the internal resistances of the electrodes and have to be taken into account for any measurement or analysis.
To address these problems, numerous measurement methods have been proposed. For example, EP 0 497 994 A1 discloses a method and device for measuring the resistance of the indicating electrode. The disclosed method proposes the use of an alternating test voltage for exciting the connecting cable and the electrode. The signal which relates to the alternating test voltage is separated from the potentiometric measurement signal by a low pass filter and a subtracting unit and is split in the corresponding real and imaginary parts by multiple synchronous rectifiers. However, this method can involve multiple expensive components such as synchronous rectifiers, which can have a significant cost increase, especially in the case of measurement devices with multiple electrodes.